Air filters of the so-called HEPA type are required to operate with a minimum 99.95% efficiency in removing airborne particles. In a popular construction of such filters, a filter core is formed of a continuous sheet of filter media folded in zig-zag fashion to form a plurality of accordion pleats of uniform size with thin, corrugated, spacer sheets placed between the opposing walls of each pleat. The filter core is supported by a surrounding, box-like frame with which the media is sealingly engaged about its entire periphery, including both pleat ends and the entire length of both media edges. The integrity of the seal between filter core and frame is critical in preventing unfiltered air from bypassing the filter.
In some applications, air filters of this type are installed in locations where they are exposed to high temperatures, e.g., on the order of 200-500° C. Problems which have been encountered with typical filter assemblies employed under such conditions include separation of and consequent air leakage between sealers and frame members, cracking of sealers, and powdering or crumbling of adhesive. These problems may be due both to the high temperatures and to poor structural strength of the filter assembly, resulting from dissimilar rates of thermal expansion, poor adhesion and low strength. Problems are particularly noticeable in applications where filter are subjected to extended periods of repeated cycling between low temperature (for example about 25° C.) and high temperature (for example in the range of 200-500° C.).
An early approach to the problem of providing the required sealing between the filter core and frame which is capable of withstanding high temperatures is that disclosed in U.S. Pat. No. 2,884,091 of Baldwin. In this construction, a resilient mat or blanket of fine glass fibers is compressed between the filter core and a surrounding metal frame. However, after prolonged exposure to temperature approaching 1000° F. the glass fibers begin to anneal, causing the mat to lose resiliency and eventually resulting in air leakage between the filter core and frame. Another filter construction intended for high temperature applications is that disclosed in U.S. Pat. No. 4,199,387 of Hladik, involving the application of a ceramic adhesive with a trowel to the zig-zag edges of the media and interposed spacer members. With this method, it is difficult to obtain a uniform sealant depth and uniformity, which could lead to leakages or undue blockage of media.
Filter constructions and methods of assembly which include immersion of the zig-zag edges of the media in a liquid sealant which later hardens to form a seal between the media and frame are shown in U.S. Pat. No. 3,581,479 of Goulet, and U.S. Pat. No. 4,227,953 of Wasielewski and Hladik. In the former, a groove or channel is formed across the inner surfaces of the top and bottom frame members for insertion of an elongated nozzle through which a liquid adhesive is injected as the nozzle is withdrawn from the groove. The assembly method of the latter patent involves filling shallow pans with the liquid adhesive and immersing therein the zig-zag edges of the pleated media. The bottom members of the shallow pans become the ends of the filter casing, and side members are adhesively sealed to the end media pleats and attached to the end members of the casing.
In order to provide HEPA filters in high filter classes, such as e.g. HEPA class H13 or higher, which filters are capable of retaining their high filtering efficiency over an extended period at high temperature, and particularly when subjected to repeated cycling between low temperature (for example about 25° C.) and high temperature (for example in the range of 200-500° C.), new filter constructions and manufacturing methods are required.